Summary
The effect of different muscle shortening velocity was studied
during cycling at a pedalling rate of 60 and 120 rev.min-1 on
the [K+]v in 21 healthy young men (aged 22.5±2.2 years, body
mass 72.7±6.4 kg, VO2max 3.720±0.426 l . min-1) performing an
incremental exercise test until exhaustion. The power output
increased by 30 W every 3 min, using an electrically controlled
ergometer Ergoline 800S (see Zoladz et al. J. Physiol. 488:
211-217, 1995). The test was performed twice: once at a cycling
frequency of 60 rev.min-1 (test A) and a few days later at
frequency of 120 rev.min-1 (test B). At rest and at the end of
each step (i.e. the last 15 s) antecubital venous blood samples
for [K+]v were taken. Gas exchange variables were measured
continuously (breath-by-breath) using Oxycon Champion Jaeger.
The pre-exercise [K+]v in both tests was not significantly
different amounting to 4.24±0.36 mmol.l-1 in test A, and
4.37±0.45 mmol.l-1 in test B. However, the [K+]v during cycling
at 120 rev.min-1 was significantly higher (p<0.001, ANOVA for
repeated measurements) at each power output when compared to
cycling at 60 rev.min-1. The maximal power output reached 293±31
W in test A which was significantly higher (p<0.001) than in
test B, which amounted to 223±40 W. The VO2max values in both
tests reached 3.720±0.426 l.min-1 vs 3.777±0.514 l.min-1. These
values were not significantly different. When the [K+]v was
measured during incremental cycling exercise, a linear increase
in [K+]v was observed in both tests. However, a significant
(p<0.05) upward shift in the [K+]v and a % VO2max relationship
was detected during cycling at 120 rev.min-1. The [K+]v measured
at the VO2max level in tests A and B amounted to 6.00±0.47
mmol.l-1 vs 6.04±0.41 mmol.l-1, respectively. This difference
was not significant. It can thus be concluded that a) generation
of the same external mechanical power output during cycling at a
pedaling rate of 120 rev.min-1 causes significantly higher [K+]v
changes than when cycling at 60 rev.min-1, b) the increase of
venous plasma potassium concentration during dynamic incremental
exercise is linearly related to the metabolic cost of work
expressed by the percentage of VO2max (increase as reported
previously by Vollestad et al. J. Physiol. Lond. 475: 359-368,
1994), c) there is a tendency towards upward shift in the [K+]v
and % VO2max relation during cycling at 120 rev.min-1 when
compared to cycling at 60 rev.min-1.